1. Field of Invention
The present invention relates to an equalization method and devices, and more particularly relates to an adaptive blind equalization method and devices to compensate for channel distortion in the 8-level vestigial sideband (8-VSB) high definition television (HDTV) transmission systems.
2. Description of Related Art
The US has adopted the high definition television (HDTV) system proposed by the Grand Alliance (GA), i.e. the American ATSC (Advanced Television Systems Committee) system. For terrestrial broadcast, the GA adopts digital 8-level vestigial sideband (8-VSB) as its modulation method. The corresponding data frame structure is depicted in FIG. 1. Each frame 100 consists of two fields; one field sync segment 102 and three hundred twelve data segments 104 comprise each field. Each segment begins with 4 binary-level symbols (segment sync) 106 having a regular pattern of +5, −5, −5, +5. In a data segment 104, the other eight hundred twenty eight symbols 108 are data symbols resulting from forward error correction (FEC) coding and having values randomly in {±1, ±3, ±5, ±7}. In the field sync segment 102, the 828 symbols 108 mostly comprise binary {±5} pseudo random (PN) sequences which are known to the receiving end. These PN sequences can be used to train the coefficients (tap weights) of the equalizer such that the equalizer can compensate for the intersymbol interference (ISI) caused by the multi-path propagation of the signal.
A simplified TV transmission example is described as follows. A series of images are firstly encoded into a data stream by an encoder. After encoding the series of images, the data stream is then transmitted with a transmitter. In the case of typical TV transmission, a RF transmitter is used for transmitting data, and another RF receiver is utilized for receiving a corresponding data stream. After receiving the data stream, a decoder is used for decoding the data stream. Finally, an image series is restored in the receiver side and shown to people.
Due to obstacles between the transmitter and the receiver, the electromagnetic wave carrying the HDTV data may suffer the so-called multi-path propagation effect. As a result, data transmitted at different times may arrive simultaneously at the receiver. An example of multi-path propagation is shown in FIG. 2. In FIG. 2, a data stream transmitted by the transmitter 202 propagates through some paths designated as “path 1” 212, “path 2” 214, and “path 3” 216 simultaneously. Because “path 1” 212 is longer than “path 2” 214 and “path 3” 216 is shorter than “path 2” 214, some data transmitted at different times are received by a receiver 204 at the same time and these data interfere with one another. Such multi-path propagation of signals causes the so-called inter-symbol interference (ISI).
To solve such problems, an equalizer at the receiving end is necessary to achieve satisfactory HDTV reception. Due to the long delay spread of multi-path channels, the equalizer requires hundreds of coefficients for adequate compensation. Compared with such a large amount of equalizer coefficients, the training sequences are too short for the coefficients to converge within one field sync segment when an adaptive algorithm, for example the least-mean-squared (LMS) algorithm, is used for tap weights adjustment. Therefore, equalizer coefficient adaptation in the data phase is very important to the system performance. The present invention is focused on the tap weight adjustment in the data phase.